A system has been under development in this laboratory to simplify study of human cell-surface antigens by procedures which permit study of antigens due to individual chromosomes. Production of human-chinese hamster somatic cell hybrids containing specific human chromosomes is achieved by carrying out fusion between selected Chinese hamster auxotrophic mutants and human cells. These hybrids which rapidly lose unnecessary human chromosomes can then be tested with antisera produced by immunization of experimental animals with a variety of normal and malignant human cells. Single cell plating procedures have been developed which make it possible to test for the presence of specific human antigens without complications due to the presence of the Chinese hamster cell antigens. We plan to continue our studies of the human antigens expressed by hybrids containing chromosome 11 which include 1) genetic and biochemical characterization, 2) regional gene mapping and 3) determination of their distribution on normal and malignant tissues. Similar studies will be undertaken using a hybrid containing human chromosome 21. Studies of these kinds should make possible rapid and definitive identification of a large variety of human cell-surface antigens which may influence malignant behavior of human cells and may help establish genetic relationships of antigens involved in the malignant response.